The invention relates generally to methods and compositions for cleaning polyethylene terephthalate containers. More specifically, the invention relates to methods and compositions which remove mold, present on polyethylene terephthalate containers, with reduced hazing.
As with many industries, the drive towards economy has also effected the beverage industry and has resulted in certain real changes in the way in which beverages are bottled, distributed and dispensed. In the last decade the beverage industry has seen a switch from glass to plastic containers. Plastic containers may be made from any number of materials depending on the application. One material is polyethylene terephthalate, xe2x80x9cPETxe2x80x9d. Two types of PET bottles that are commonly used are single trip and multi-trip bottles. Single trip bottles are those which are filled, used, and then discarded. Multi-trip bottles are collected and reused and must be washed before refilling.
PET bottles offer several advantages over glass. Their light weight reduces freight costs. When dropped onto a hard surface they do not shatter like glass and generally do not break. Conveyor wear caused by the containers in the packaging plant is also reduced. The closure is also generally reusable after a bottle has been opened.
Disadvantages are that PET bottles are easily scratched, and susceptible to chemical attack when they are washed. PET containers also do not tolerate conditions above 60xc2x0 C. Exposing them to higher temperatures than 60xc2x0 C. causes deformation and/or shrinkage of the bottles.
Recycling of PET containers was recently approved by the FDA in the USA to permit turning used containers into new ones. Because new resin costs 50 to 75 cents per pound, recycling used bottles is economically attractive. In time, glass soft drink bottles are expected to disappear from the market. This heightens the relevance of PET container processing even further.
The cleaning of PET bottles takes place over a series of steps using caustic immersion tanks and spray wash stages in a bottlewasher. In the wash tanks, product residue, dirt, labels and labeling adhesive are removed. Because the surface of PET bottles is hydrophobic, cleaning them is more difficult than glass bottles. Also, the lower washing temperature decreases the chemical activity of the bottlewashing solution.
Two principle problems in the cleaning of PET bottles are touched on by Laufenberg et al., xe2x80x9cCleaning, Disinfecting, and Transporting Pet Returnablesxe2x80x9d, Brew Bev. Ind. Int. 1, 40-4 (0 ref.) January, 1992.
In this article, the author outlines the susceptibility of the PET bottles to corrosion or hazing. Hazing results from the chemical etching of the surface of the PET container by the caustic present in the wash bath.
An especially challenging problem which affects reusable PET bottles is the occurrence of mold in returned bottles. To simply discard all bottles from which mold can not be removed is prohibitively expensive. Reject rates of 40 to 50% have occurred at certain times of the year in countries located in tropical climates.
The necessary cleaning temperature for PET bottles is 60xc2x0 C. or less due to the glass transition temperature of PET. If exceeded, PET bottles deform and shrink. The cleaning power of a bottlewashing solution at 60xc2x0 C. is only one quarter that at 80xc2x0 C. Bottles returned with product residue, i.e., those bottles that have not been rinsed, are almost always contaminated with microbiological forms of life. The bottle washing solution eliminates the presence of microbiological forms of life such as bacteria, spores, molds, and yeasts present in the bottle. However, at the reduced temperature of 60xc2x0 C., molds often present a persistent problem in the cleaning and reuse of PET bottles.
PET bottles simply cannot be washed like glass. Glass bottles are normally washed at 80xc2x0 C. Glass can also be washed with a relatively high concentration of caustic. While glass may be washed with up to 5.0% caustic, as little as 1.5% caustic can cause hazing in a PET container. With glass the washing temperature, the caustic concentration, and the washing time may be adjusted to allow for variability within the environment. In contrast, PET containers cannot withstand high levels of any of these variables.
While various alternatives have been proposed such as lowering the level of caustic, there remains a need in the industry for compositions and methods which allow for the efficient cleaning of PET containers and multiple reuse events.
The first aspect of the invention is a method for cleaning a polyethylene terephthalate container. The method includes combining a first concentrate with a second concentrate in an alkaline wash solution. The first concentrate includes from about 0.3 to 25 wt-% of a surfactant and from about 5 to 30 wt-% of an acid. The second concentrate includes from about 8 to 60 wt-% of a builder. The method further includes the step of contacting the PET container with the wash solution wherein the removal of soil is undertaken with minimal hazing of the container.
An additional aspect of the invention is a method for cleaning polyethulene terephihalate containers which includes contacting the container with an alkaline wash solution having a temperature ranging from about 50xc2x0 C. to 60xc2x0 C. The alkaline wash solution is formulated from a first concentrate, a second concentrate, an alkalinity source, and a balance of water. The first concentrate includes a nonionic surfactant, a first builder, and acid in an amount effective to provide a phase stable solution. The second concentrate includes a nonionic surfactant and a second builder. The first and second concentrate are present in the wash solution in a concentration ranging from about 0.5 wt-% to 1.2 wt-%, and the first nonionic surfactant has a cloud point ranging from about 5xc2x0 C. to 60xc2x0 C.
A further aspect of the invention is an alkaline wash solution for cleaning polyethylene terephthalate bottles. The wash solution includes from about 1 to 5 wt-% of a source of alkalinity, from about 480 to 4000 ppm of a builder, from about 6 to 500 ppm of a surfactant, and from about 20 to 800 ppm of a coupler.
The invention is compositions and methods for cleaning polyethylene terephthalate (PET) bottles with enhanced removal of mold and reduced hazing. In addition to the 1 to 3 wt-% caustic commonly used in PET bottle washing procedures, the compositions of the invention include surfactants and builder combination, which heightens cleaning and removes mold.
Hazing generally results from chemical etching caused by caustic present in the wash solution. Hazing is a clouding or dulling of the PET container surface which detracts from the aesthetic character of the container. Surprisingly, it has been found that by using surfactants with appropriate cloud points, hazing may be substantially reduced. Preferably, PET containers treated with the wash solution of the invention are substantially free of hazing.
Further, mold growth, particularly in the inside of returned PET bottles, proves to be a major challenge in bottle washing. Molds are very difficult to remove, even with a solution having as much as 3% caustic. Good cleaning usually removes most of the organic components of mold. However, the inorganic residues of mold may remain on the PET container surface. This may cause a problem similar to water spots. This condition gives a positive test result by methylene blue staining, (Industrial Code of Practice for Refillable PET Bottles, Edition 1 (1993-1994 UNESDA/CESDA, pg. V-18). The compositions and methods of the invention substantially remove soils and both the organic and inorganic residues of mold.
Generally, the wash solution of the invention is formulated from two concentrate compositions. These two concentrate compositions are combined in an aqueous wash solution with an alkalinity source, before use. These concentrate compositions generally comprise surfactants, an acid, builders such as sequestrants and chelating agents, coupling agents, and various other adjuvants.
A. The Surfactant System
Generally, the compositions of the invention comprise surfactants to facilitate low foaming cleaning, and prevent hazing of the PET container. Any number of surfactants may be used in accordance with the invention including nonionic surfactants, anionic surfactants, amphoteric surfactants, and mixtures thereof.
Nonionic surfactants encompass a wide variety of polymeric compounds which include specifically, but not exclusively, ethoxylated alkylphenols, ethoxylated aliphatic alcohols, ethoxylated amines, ethoxylated ether amines, carboxylic esters, carboxylic amides, and polyoxyalkylene oxide block copolymers.
Preferably, nonionic surfactants are used in the invention such as those which comprise ethylene oxide moieties, propylene oxide moieties, as well a mixtures thereof, and ethylene oxide-propylene oxide moieties in either heteric or block formation. Additionally useful in the invention are nonionic surfactants which comprise alkyl ethylene oxide compounds, alkyl ethylene oxide-propylene oxide compounds and alkyl ethylene oxide-butylene oxide compounds, as well as mixtures thereof. The ethylene oxide propylene oxide moiety and ethylene oxide-butylene oxide moiety may be in either heteric or block formation. Also useful in the invention are nonionic surfactants having any mixture of combination of ethylene oxide-propylene oxide moieties linked to an alkyl chain where the ethylene oxide and propylene oxide moieties may be in any randomized or ordered pattern and of any specific length. Nonionic surfactants useful in the invention may also comprise randomized sections of block and heteric ethylene oxide propylene oxide, or ethylene oxide-butylene oxide.
Preferred nonionic surfactants include alkylphenols, alcohol ethoxylates, and block copolymers of ethylene oxide and propylene oxide.
Examples of nonionic surfactants found useful in the invention include (EO)/(PO) block copolymers having at least about 3 moles (EO) and at least about 15 moles (PO); aryl or aliphatic ethoxylates having at least about 3 moles (EO) which may or may not be capped with methyl, butyl, or benzyl moieties; aryl or aliphatic ethoxylate-propoxylate copolymers having at least about 2 moles of (EO) and from about 4 moles of (PO) and which may also be capped with methyl, butyl or benzyl; and aryl or aliphatic ethoxylate-butoxylate copolymers having at least about 2 moles of (EO) and about 4 moles of (BO) and which may also be capped with methyl, butyl or benzyl. The aliphatic group may comprise any branched or linear C8-C24 moiety. The aryl group may generally comprise aromatic structures such as benzyl. An HLB value of 4 to 13 may also be used to characterize surfactants useful in the invention.
Representative nonionics which are useful in the invention include EO/PO block copolymers available from Henkel KGaA; Pluronic L62 and L44 which are EO/PO block copolymers available from BASF; Tergitol 15-S-3, TMN3, TMN10 which are ethoxylated alcohols available from Union Carbide; Surfonic L24-1.3 which is a linear alcohol ethoxylate available from Texaco Chemical Co.; nonyl phenol ethoxylates such as NPE 4.5, NPE 9, and Surfonic N120 available from Texaco Chemical Co.; ethoxylated alkyl amines such as ethoxylated coco amine available from Sherex Chemical Co. as Varonic K-215; an alkyl ethoxylated carboxylic acid such as Neodex 234; and benzylated alcohol ethoxylates and EO/PO block copolymers among other nonionic surfactants.
Also useful in the invention are low foaming surfactants which may oil out of the wash solution at a temperature of 59xc2x0 C. or less. Preferably, the surfactant system comprises surfactants having a cloud point of about 5xc2x0 C. to 60xc2x0 C., preferably from about 10xc2x0 C. to 50xc2x0 C., and more preferably of about 10 to 20xc2x0 C. so that in the alkaline wash solution, the surfactants will oil-out or film and deposit on the PET container surface providing protection against hazing.
One preferred line of surfactants includes Dehypon LT104 which is a C12-18 fatty alcohol (EO)10 butyl capped and LS24 which is a C12-14 fatty alcohol ((EO)2 (PO)4) both available from Henkel Canada Ltd.
Anionic surfactants may also be used in the invention. Typical commercially available anionic surfactants provide either a carboxylate, sulfonate, sulfate or phosphate group as the functional anion. We have found that carboxylate based anionic surfactants such as alcohol ethoxylate carboxylates reduce hazing of the container. A commercial source of this type of surfactant is Neodox 234-4(trademark) available from Shell Chemical Co.
Amphoteric surfactants may also be used in the invention. Such amphoteric surfactants include betaine surfactants, sulfobetaine surfactants, sarcosinate surfactants, amphoteric imidazolinium derivatives and others. Certain surfactants found useful in hazing reduction include cocoyl and lauroyl sarcosine/sarcosinates such as Hamposyl C and L available from Hampshire Chemical Co.
B. Acid
The composition of the invention may also comprise an acid source. The acid functions to stabilize the surfactant system so that prior to mixing in the wash solution, the concentrate is a true phase stable solution. Once added to the alkaline wash solution the acids are neutralized, become salts, and provide heightened cleaning efficacy and retard the formation of scaling on washing machine components. Generally, the acid may be any number of organic or inorganic acids.
Inorganic acids useful in the composition and the invention include phosphoric acid, polyphosphoric acid or acidic pyrophosphate salts, among others. Organic acids useful in the invention include mono and polycarboxylic acids such as acetic acid, hydroxyacetic acid, citric acid, gluconic acid, glucoheptanoic acid, lactic acid, succinic acid, malonic acid, glutaric acid, and mixtures thereof.
C. Builders
The composition of the invention may also comprise a builder. Builders, i.e., sequestrants and chelating agents, retard the precipitation of scale onto the side walls of the PET container and the bottle washing machine. Builders also facilitate soil suspension, bind hardness ions and, in turn, enhance cleaning, during the washing process. In accordance with one embodiment of the invention the first concentrate may contain a first builder and the second concentrate may contain a second builder.
Builders which may be used in accordance with the invention include sequestrants such as phosphonates, phosphinates, acrylates and polyacrylates, and polycarboxylates, among others. Also useful as builders are maleate polymers and copolymers of maleate and acrylate; salts such as polyaspartic and polyglutaric acid salts; erythorbic acid; polyacrylamidopropyl sulfonate; and phosphino carboxylic acid, among others.
Water soluble acrylic polymers which may be used include polyacrylic acid, polymethacrylic acid, acrylic acid-methacrylic acid copolymers, hydrolyzed polyacrylamide, hydrolyzed methacrylamide, hydrolyzed acrylamide-methacrylamide copolymers, and mixtures thereof. Water soluble salts or partial salts of these polymers such as their respective alkaline metal (for example sodium or potassium) or ammonium salts can also be used.
Also useful as builders are phosphonic acids and phosphonic acid salts. Such useful phosphonic acids include, mono, di, tri, tetra and, penta phosphonic acids which can contain groups capable of forming anions under alkaline conditions.
The phosphonic acid may also comprise a lower molecular weight phosphonopolycarboxylic acid such as one having about 2-4 carboxylic moieties and about 1 to 5 phosphonic acid groups. Such acids include 1 phosphono-1-methylsuccinic acid, phosphonosuccinic acid and 2-phosphonobutane-1,2,4 tricarboxylic acid.
Preferred sequestrants include the Dequest(copyright) sequestrants available from Monsanto Co. including Dequest 2006(copyright) which is amino tri(methylene phosphonic acid) pentasodium salt; Dequest 2010(copyright) which is 1-hydroxyethylidene-1,1-diphosphonic acid; Bayhibit AM(copyright) available from Mobay Chemical Co. which is 2-phosphonobutane-1,2,4-tricarboxylic acid; Dequest 2000(copyright) which is aminotri(methylene phosphonic acid); and Belsperse 161(copyright) from Ciba Geigy which is a phosphino polycarboxylic acid.
The builder present in either concentrate may also be a chelating,agent. Unlike a sequestrant, the chelating agent tends to bind alkali earth metals present in the wash solution and hold these compounds in solution. It is believed that mold uses the organic portion of nutrients leaving behind inorganic salts. As a result, the ineffective removal of mold is often indicated by inorganic salts which are left behind on the surface of the PET container. The chelating agent removes these inorganic salts that are found underneath the mold.
The number of bonds capable of being formed by a chelating agent upon a single hardness ion is reflected by labeling the chelating agent as bidentate (2), tridentate (3), tetradendate (4), etc. Any number of chelating agents may be used in accordance with the invention. Representative chelating agents include salts of amino carboxylic acids, phosphonic acid salts, water soluble acrylic polymers, among others.
Suitable amino carboxylic acid chelating agents include N-hydroxyethyliminodiacetic acid, nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), N-hydroxyethyl-ethylenediaminetriacetic acid (HEDTA), and diethylenetriaminepentaacetic acid (DTPA), as well as isoserine-N,N-diacetic acid, beta alanine N,N-diacetic acid, sodium glycolate, and tripolyphosphate, among others. In accordance with one useful aspect of the invention the second builder present in the second concentrate comprises an amino carboxylic acid chelating agent, preferably of ethylene diamine tetracetic acid or salts thereof.
D. Couplers
The composition of the invention may also include a coupling agent. The coupling agent functions to stabilize the concentrate composition so that it is a true phase stable solution.
To this end, any number of organic coupling agents may be used including sulfates, sulfonates, as well as monofunctional and polyfunctional alcohols. Preferred coupling agents include sulfonate and sulfate compounds such as sodium xylene sulfonate, sodium cumene sulfonate, sodium toluene sulfonate, 2-ethylhexyl sulfate, alkyl diphenyl oxide disulfonate where the alkyl group is either a branched C12 or a linear C10, sodium alkyl naphthalene sulfonate, and sodium octane sulfonate and disulfonate, and mixtures thereof.
Those coupling agents which have been found useful include linear alkyl alcohols such as, for example, ethanol, isopropanol, and the like. Also useful are polyfunctional hydroxy compounds such as alkylene glycols like hexylene glycol and propylene glycol; phosphate esters including Gafac RP710 from Rhone-Poulence Chemicals, and Triton H-66 from Rohm and Haas Co.
E. Adjuvants
The compositions and methods of the invention may use any number of other adjuvants such as added nonionic surfactant defoamers such as those disclosed in U.S. Pat. No. 5,516,451 to Schmitt et al. which is incorporated herein by reference. Tracing compounds such as potassium iodide, colorants and dyes, fragrances, and preservatives, among other constituents are also useful in the invention.
The method of the invention provides heightened cleaning efficacy of PET containers, removing soils, inorganic salts, and molds while retarding hazing of the containers. This result is obtained by formulating a first acidic concentrate with a high concentration of surfactant and a separate second concentrate having a high concentration of builder.
In accordance with one aspect of the invention the first and second concentrates may be incompatible if mixed separately from a wash solution. Incompatibility in this context stems from different pH requirements of the two different concentrates. The first concentrate may generally have an acidic pH of less than about 2 to maintain the solubility of the surfactant system. The pH of the second concentrate is selected to provide complete solubility of the builder and is generally alkaline. Combination of the two concentrates before dilution in the wash solution may result in phase separation of the surfactant system or the builder depending on pH.
In use the two concentrates are combined in an alkaline wash system to provide heightened cleaning efficacy with good building efficacy. Illustrative concentration ranges for each of the two concentrates are provided below:
In use this system is diluted into a wash solution comprising from about 0.1 wt-% to 0.8 wt-%, preferably from about 0.2 wt-% to 0.3 wt-% of the first concentrate and from about 0.2 to 1.2 wt-%, and preferably from about 0.4 wt-% to 0.8 wt-% of the second concentrate. The ratio of the first concentrate to the second concentrate in the alkaline wash solution generally ranges from about 0.1:0.5 to 0.1:1.0, and preferably from about 0.1:0.2 to 0.15:0.3. Generally the alkaline wash solution may have a total of from about 0.3 to 2.0 wt-% and preferably from about 0.5 to 1.2 wt-% of both concentrate 1 and concentrate 2.
In accordance with one preferred aspect of the invention, the wash solution comprises at least about 1000 ppm EDTA, at least about 5 ppm of a phosphonate compound, and at least about 100 ppm of a gluconate compound.
The washing of PET containers generally takes place over a number of steps. The PET containers are emptied and pre-rinsed, then soaked in the wash solution. The wash solution generally has anywhere from 1.0 wt-% to 5 wt-% and preferably from 1.5 wt-% to 3 wt-% caustic (NaOH). Into this system is mixed Concentrate 1 and Concentrate 2 into which the PET containers are applied. Washing tends to take place over a time period which ranges from about 7 to 20 minutes. The wash temperature is about 59xc2x0 C. xc2x11xc2x0 C. The containers then pass through a weak caustic stage where water is run into the tank to continue cleaning and begin rinsing by reducing the sodium hydroxide concentration. The caustic concentration may be maintained by a conductivity controller. The containers then pass through at least three rinse stages which sequentially rinse the containers with warm water and cold water. The final rinsing takes place with potable water after which time the containers are turned upright, inspected, and filled.